New Zealand Police Sergeant Louis Ott is dealing with the fourth stabbing in an hour. It's a wet Saturday night in south Auckland, and Ott and constable Brent Stevenson are questioning a shivering youth as blood congeals in a gash to his nose. "I fell over," says the boy, blinking in the light of Ott's torch. "C'mon, bro," says Ott. "How would you feel if someone got badly stabbed tonight, and died, by the same people that did this?" The teenager, who is mysteriously wearing a clean shirt turned inside out, admits that the wound, from a box cutter, was received...